The present invention relates to devices for locating eccentricities in cartridge casings, bullets, and cartridges for firearms.
All shooters, whether they are police officers, olympic shooters, or weekend enthusiasts, have one common goal: hitting their target accurately and consistently. Accuracy and consistency in shooting depend largely on the skill of the shooter and the construction of the firearm and ammunition.
The accuracy of a firearm can be enhanced by the use of precisely-made components, including precisely-made ammunition. It is well known in target shooting that using ammunition in which the propellant weight and type, bullet weight and dimensions, and cartridge dimensions are held within very strict limits, and are thus consistent can improve accuracy in shooting. The wider the variation or impreciseness of these individual factors, the greater the loss of accuracy. Such loss of accuracy produces wide variations in bullet trajectory, which in target shooting produces a large pattern or grouping of bullet holes in the target (and a lower score than would be expected if the group were smaller or "tight"). The variations in the physical components of the ammunition should therefore be considered in order to obtain optimum accuracy.
As a practical matter, commercially available ammunition or ammunition components, like casings, bullets, primers, and propellant, are all made within certain tolerances set by their manufacturers. Therefore, while these components may be within the manufacturing tolerance, some of the components may be near the optimal specification while others may be very close to the outside tolerance.
For example, cartridge casings are optimally cylindrical in shape. Thus, if one imagines a longitudinal axis passing through the center of casing from the head of the casing extending out of the mouth of the casing, a cross-section of the casing taken anywhere along the axis, perpendicularly to the axis, would reveal that the axis is in the center of a circle, if the casing is optimally shaped. That is, the wall of the casing is "concentric." However, perfect concentricity is rare; casings, bullets, and cartridges may exhibit one or more eccentricities. Thus, casings may be produced which have a "thin" side or a "thick" side, or bullets may be produced which deviate from a perfectly circular cross section to produce a "high" area where two halves of a mold joined. Such variations from the optimum are called "eccentricities." As pointed out by Creighton Audette in "Overlooked Aspects of the Cartridge Case," Precision Shooting, December 1981, Vol. 27, No. 8, one must consider the eccentricities if accuracy in shooting is to be attained.
"Runout" is a term which is often used to describe eccentricities which affect the outer (external) surface of a cartridge casing, bullet, or other generally cylindrical object. Runout is a deviation from the midline axis (also known as the central longitudinal axis or the geometric axis) of the outside surface of a cylinder. For example, cartridge casing "runout" can involve a "banana shape" curve in which the casing is not a perfectly straight cylinder, but bends longitudinally. With this kind of runout, each cross section may be perfectly circular, but eccentric or offset relative to the geometrical axis of the cartridge as a whole. "Banana shape" runout is typically seen only in fired cartridge casings (which are commonly cleaned, resized and reloaded), and typically becomes more pronounced with every firing.
"Egg-shape," or out-of-round runout occurs when one transverse cross section of a casing, bullet or cartridge is not circular. This eccentricity is typically local, and may occur at any one cross section independently of its occurrence at any other cross section. Having found egg shape at one point, one cannot predict what various degrees, shapes or orientations of deformity might be found elsewhere.
Head runout occurs when the head on a casing is not "square." This condition occurs when the head of the casing (or more commonly, a portion of the head of the casing) is not perpendicular to the central longitudinal axis of the casing.
Yet another troublesome eccentricity occurs when the casing wall thickness is not uniform. When this occurs, a portion of the casing wall is thinner. Because the outer surface of the casing may be uniform, checking for runout will not reliably determine variations in casing wall thickness. If the casing is thinner in a certain area, that section of the wall may unevenly stretch upon firing in both width and length, and the whole casing may, after several uses, be distorted into a banana-like shape.
All of these eccentricities effect accuracy in shooting. With runout, the bullet can be misaligned off axis in the bore when the cartridge is chambered in the firearm. If the casing thickness varies, for example in the neck region where the bullet is lodged, the bullet can be laterally offset and again may be positioned in the bore off axis from the optimum position when chambered. As noted above, very small errors will spread the bullet trajectories. For accurate shooting, the casing, bullet and loaded cartridge should be true within 0.001 inch or less.
In a cartridge having a bullet and casing which may each have one or more different eccentricities, random orientation of the eccentricities will cause a loss of accuracy (larger group size) when shooting at a target. A cartridge with one or more randomly oriented eccentricities can have unpredictable ballistics. Cartridge runout (out-of-roundness, bending, or lack of symmetry about the central axis of the cartridge) in either the bullet or the casing will spread the bullet trajectories if the departure from truly cylindrical shape is as little as a thousandth of an inch. If one could determine the location of the eccentricities, it would be possible to orient the eccentricities and mark or "index" the loaded cartridge. As explained by Creighton Audette, indexing allows the shooter to consistently align the eccentricities in a loaded cartridge in the chamber of a firearm, thus offsetting any inaccuracy due to the eccentricities. As noted above, eccentricities as small as 0.001 inch can effect accuracy. Such small errors are not detectable with the naked eye, and are difficult to detect with ordinary equipment.
Thus, the need exists for a fixture which can reliably check cartridge casings, bullets, and loaded cartridges for runout and check cartridge casings for variations in wall thickness. Such a fixture should be accurate, easy to use, inexpensive and rugged.